1. Field of the Invention
This invention relates to the synthesis and in vivo application of compounds which have antibiotic activity against microbes that synthesize mycolic acid, including Mycobacterium sp., particularly drug resistant Mycobacterium strains, and to the use of these compounds to treat any susceptible pathogenic microorganism or parasite.
2. Review of Related Art
The emergence of multiply drug resistant (MDR) strains of Mycobacterium tuberculosis and other atypical mycobacteria which infect immunocompromised patients (e.g., AIDS patients) highlights the need for continued antibiotic development.
Mycobacterium sp. synthesize a multitude of complex lipids and glycolipids unique to this genus, making these biochemical pathways attractive targets for drug therapy (Bloch, K., “Control mechanisms for fatty acid synthesis in Mycobacterium smegmatis,” Adv. Enzymol. 45:1-84, 1977; Brennan, P. J., and Nikaido, H., “The envelope of mycobacteria,” Ann. Rev. Biochem. 64:29-63, 1995). The β-ketoacyl synthase (KS) of particulate Type II fatty acid synthases or the corresponding domain of the polyfunctional Type I fatty acid synthases catalyzes the critical two-carbon homologation during buildup of the growing fatty acid chain. This process typically gives acids of length C16 to C18. In chain elongation of normal fatty acids, carried out for example by mycobacteria, CoA and/or acyl-carrier protein (ACP) thioesters of these acids are further reacted with malonyl-CoA to greatly extend their length to 60-90 carbons. These high molecular weight acids are known collectively as mycolic acids.
Mycolic acids are a group of complex, long, branched chain fatty acids that are vital for the growth and survival of mycobacteria. Mycolic acids comprise the single largest component of the mycobacterial cell envelope. Little is known about the nature of the biosynthetic enzymes involved, but evidence suggests some similarity to conventional fatty acid synthases (Bloch, 1977; Brennan, et al., 1995). These unusually long lipid molecules form a waxy coat of limited permeability.
The presence in mycobacteria of particular modified fatty acids having complex and well-organized structures presents a potentially attractive target for drug design (Young, D. B., and Duncan, K., “Prospects for new interventions in the treatment and prevention of mycobacterial disease,” Ann Rev. Microbiol. 49:641-673, 1995). It has been suggested that isoniazid inhibits mycolic acid synthesis as its potential mechanism of action (Takayama, K., Wang, L., and David, H. L., “Effect of isoniazid on the in vivo mycolic acid synthesis, cell growth, and viability of Mycobacterium tuberculosis,” Antimicrob. Agents Chemother., 2:29-35, 1972; Takayama, K., Schnoes, H. K., Armstrong, E. I., and Booyle, R. W., “Site of inhibitory action of isoniazid in the synthesis of mycolic acids in Mycobacterium tuberculosis,” J. Lipid Res., 16:308-317, 1975; Quemard A., Dessen A., Sugantino M., Jacobs W. R., Sacchettini J. C., Blanchard J. S. “Binding of catalase peroxide-activated isoniazid to wild-type and mutant Mycobacterium tuberculosis enoyl-ACP reductases,” J. Am. Chem. Soc., 118:1561-1562, 1996; Baldock C., Rafferty J. B., Sedenikova S. E., Baker P. J., Stuitje A. R, Slabas A. R., Hawkes T. R., Rice D. W. “A mechanism of drug action revealed by structural studies of enoyl reductase,” Science, 274:2107-2110, 1996; Quemard A., Sacchettini J. C., Dessen A., Vilcheze C., Bittman R., Jacobs W. R., Blanchard J. S., “Enzymatic characterization of the target for isoniazid in Mycobacterium tuberculosis, Biochemistry, 34:8235-8241, 1993; Msluli, K., D. R. Sherman, M. J. Hickey, B. N. Kreiswirth, S. Morris, C. K. Stover, and C. E. Barry, III, “Biochemical and genetic data suggest that InhA is not the primary target for activated isoniazid in Mycobacterium tuberculosis,” J. Infect. Dis., 174:1085-1090, 1996; Dessen A., A. Quemard, J. S. Blanchard, W. R. Jacobs, and J. C. Saccettini, “Crystal structure and function of the isoniazid target of Mycobacterium tuberculosis,” Science, 267:1638-1641, 1995; Banerjee, A., E. Dubnau, A. Quemard, V. Balasubramanian, K. S. Um, T. Wilson, D. Collins, G. deLisle, W. R. Jacobs, Jr., “InhA, a gene encoding a target for isoniazid and ethionamide in Mycobacterium tuberculosis.” Science, 263:227-230, 1994). This finding might be expected to stimulate a search for novel compounds that act upon the lipid synthetic pathways of mycobacteria as a fresh approach for antibiotic development. Surprisingly, however, lipid biosynthesis has not been exploited for drug development in these organisms. No drugs which specifically inhibit mycobacterial lipid synthesis have been developed other than isoniazid, and there remains a need for new drugs to treat the growing problem of multi-drug resistant mycobacteria.